Currently, direct homogeneous partial oxidation (DHPO) produces a variety of oxygenates typically consisting of alcohols and aldehydes, and in smaller concentrations carboxylic acids. Conversion of these liquid products into higher value fuels and chemicals via process integration is of great interest since process integration permits for cost reduction and therefore applicability at smaller scale.
The presented invention incorporates a gas to chemicals process with alcohol dehydration to produce dimethyl ether or an ether blend which presents more favorable properties for use as a diesel fuel substitute and additive. Advantages of process integration include the utilization of high temperatures and pressures already used by the gas to chemicals process, thus decreasing energy and equipment demands. Integration of a reactive distillation column thus facilitates separation of liquid products from the gas-to-chemicals process and also synthesizes a product of greater value.
Increasing the carbon length of the alkane feed gas is known to produce higher proportions of alcohols, aldehydes and carboxylic acids with carbon length greater than one. Many of these components have relative volatilities at standard temperature and pressure similar to that of water, complicating separations with conventional techniques. Furthermore, formaldehyde reversibly forms methylene glycol polymers that can interfere with recovery of high boiling alcohols. Therefore, there exists a need to efficiently separate these components, preferably as a readily usable product.
Pure methane gas, on the other hand, subjected to the direct homogeneous partial oxidation process produces primarily formaldehyde, methanol, and ethanol. Methanol is known, in the presence of certain catalysts, to undergo dehydration to DME. Furthermore, methanol is known to cleanly separate from a solution of higher alcohols and water under conventional distillation with reflux. In addition, ethanol is known to form an azeotrope with formaldehyde, allowing for the formaldehyde to remain with the ethanol and water as a polymerization inhibited blend. Through this property, the composition of oxygenates created by the DHPO process, upon removal of methanol, is sufficient for a commercial formalin blend. In the context of a reactive distillation column, aqueous formaldehyde is separated into formaldehyde gas and liquid water to generate an overhead product consisting of dialkyl ethers and formaldehyde gas and bottoms product composed of water.
As previously mentioned, feed gasses with a higher average molecular weight of alkane content than methane are known to produce higher carbon chain length alcohols as well as acids with correspondingly lower volatility. Methanol is also known to form dimethyoxymethane (methylal) in an acid catalyzed reaction with methanol and formaldehyde. In particular, sulfonic acid polymers such as nafion® and amberlyst® can catalyze such reactions. Other alcohols are known to form similar analogs. Said acid catalysts are also known to esterify carboxylic acids to esters and alcohols to ethers. This transformation eliminates the aldehyde azeotropes and increases the volatility of the mixture.
Dimethyl ether is known to be a substitute and blending additive for LPG and potentially diesel. It is also an intermediate in the synthesis of higher-value chemicals as well as gasoline. Dimethoxymethane as well as other dialkyl ethers exhibit similar properties with improved utility as a diesel substitute or oxygenate. Processing options include the removal of formaldehyde prior to reactive distillation. This may be achieved by reacting formaldehyde with different chemicals to produce compounds easily separated from the liquid mixture, eg. formaldehyde may react with urea to produce urea-formaldehyde. In this case, alcohols are easily separated from water and the formaldehyde-based component. Dehydration of the alcohol fraction (methanol and ethanol) would therefore result in an ether blend which presents characteristics that make the ether blend a more attractive diesel substitute or blending additive than DME alone.
Accordingly, there is a need for improved methods of making blends of ethers and other partially oxygenated compounds.